EP2774202B1 - Fuel cell including a cathode having channels - Google Patents

Description

The present invention relates to the field of fuel cells.

• une anode apte à oxyder un premier composé M1 en premiers ions M^(m+) avec m entier non-nul,
• un premier électrolyte qui est en contact avec ladite anode,
• une cathode apte à réduire un second composé N2 en seconds ions N^(n-) avec n entier non-nul,
• une membrane centrale poreuse dont une des faces est en contact avec ledit premier électrolyte, et dont la face opposée est en contact avec ladite cathode,

le premier électrolyte étant constitué d'un matériau apte à conduire les ions M^(m+), la membrane centrale étant constituée d'un matériau apte à conduire à la fois les ions M^(m+) et les ions N^(n-).In particular, the present invention relates to a fuel cell comprising

• an anode capable of oxidizing a first compound M1 to first ions M ^{(m +)} with m non-zero integer,
• a first electrolyte which is in contact with said anode,
• a cathode capable of reducing a second compound N2 into second ions N ^(n-) with n non-zero integer,
• a porous central membrane, one of the faces of which is in contact with said first electrolyte, and the opposite face of which is in contact with said cathode,

the first electrolyte consisting of a material capable of conducting the M ^{(m +)} ions, the central membrane being made of a material capable of conducting both the M ^{(m +)} ions and the N ^(n-) ions.

Fuel cells (PAC) are set to become a source of alternative energy production to those coming directly from fossil fuels, for stationary applications, but also for embedded applications (for example the automobile) in the longer term.

• avec n, m, et p entiers non-nuls.
• M1 désigne un composé d'atomes identiques ou distincts qui, en s'oxydant, donne un premier ion M^(m+) (ou m premiers ions M⁺) et m électrons.
• N2 désigne un composé d'atomes identiques ou distincts qui, par réduction en présence de n électrons, donne un second ion N^(n-) (ou n seconds ions N^-).

A PAC operates on the principle of an electrochemical and controlled oxidation-reduction of a first compound M1 and a second compound N2, with simultaneous production of electricity, compound P and heat, according to the overall chemical reaction

{n M1 + m N2 → p P}

• with n, m, and p non-zero integers.
• M1 denotes a compound of identical or distinct atoms which, by oxidizing, gives a first ion M ^{(m +)} (or m first M ⁺ ions) and m electrons.
• N2 denotes a compound of identical or distinct atoms which, by reduction in the presence of n electrons, gives a second ion N ^(n-) (or n second ions N ^- ).

It is this electricity produced that can then supply energy to a device.

For example, the CAP is such that the first compound M1 is hydrogen (H ₂ ) and the second compound N2 is oxygen (O ₂ ), and the overall chemical reaction is

{H ₂ + ½ O ₂ → H ₂ O}.

The product compound of this reaction is H ₂ O.

Below is an example of such a PAC.

Solid oxide fuel cells (hereinafter referred to as "SOFC") and proton conductive fuel cells (hereinafter referred to as PCFC) are known. .

In a SOFC, the water is formed at the anode, and discharged through the anode, resulting in undesirable dilution of the fuel (hydrogen) by the water at the anode, as well as a decrease in the catalytic activity, the water appearing on the active sites of the anode

In a PCFC, water is formed at the cathode, and discharged through the cathode, resulting in undesirable dilution of oxygen by water at the cathode, as well as a decrease in catalytic activity, the water appearing on the active sites of the cathode.

In both cases, the performance of the fuel cell is diminished, and is insufficient.

To solve this problem, a mixed conduction membrane fuel cell has been developed. The operation of such a fuel cell is recalled hereinafter with reference to the figure 4 which illustrates such a battery and which constitutes the prior art.

• Une anode 100,
• Un premier électrolyte 200,
• Une membrane centrale 300,
• Un second électrolyte 400,
• Une cathode 500.

This fuel cell, shown in section, comprises five main layers which are in pairs in contact and whose stack along a main axis A is, in this order:

• Anode 100,
• A first electrolyte 200,
• A central membrane 300,
• A second electrolyte 400,
• A cathode 500.

Anode 100 is the seat of an oxidation reaction of hydrogen:

{H ₂ → 2H ⁺ + 2 e ^- }.

H ⁺ protons thus created migrate to the central membrane 300 through the first electrolyte 200. This first electrolyte 200 is a material capable of driving H ⁺ protons.

The e ^- produced electrons flow from the outside of the fuel cell from the anode 100 through a conductor 90 to join the cathode 500 and supply it with electrons (see below).

Cathode C is the seat of an oxygen reduction reaction:

{½ O ₂ + 2e ^- → O ^2- }

O ^2- ions thus created migrate to the central membrane 300 through the second electrolyte 400. This second electrolyte 400 is a material capable of conducting O ² ions.

Thus a first face of the central membrane 300 is in contact with the first electrolyte 200, and a second face of the central membrane 300, opposite this first face, is in contact with the second electrolyte 400.

The central membrane 300 is a composite of the first electrolyte 200 and the second electrolyte 400, so as to be able to conduct both H ⁺ protons and O ^2- ions.

Within this central membrane 300, these protons H ⁺ and O ² ions react according to the following reaction to produce water:

{2H ⁺ + O ^2- → H ₂ O}

This central membrane 300 is also porous with porosities 380, in order to allow better evacuation of the water thus produced by this central layer 300.

Such a fuel cell is shown for example in the document US 2008/0213639 and, compared to single-conduction fuel cells (SOFC and PCFC), the advantage of avoiding the anode fuel dilution that occurs in SOFC and the dilution of oxygen at the cathode that occurs in a PCFC. Corrosion of the inter-connectors is also avoided and the overvoltages associated with the presence of water at the electrodes are limited in a standard SOFC and PCFC type cell.

The evacuation of water (ie evacuation which does not reduce the performance of the fuel cell) in such a mixed conduction membrane fuel cell is however not satisfactory because this evacuation is limited by the thickness and the dimensions of the porosity of the central membrane of this stack.

The present invention aims to remedy these disadvantages.

The aim of the invention is to propose a fuel cell in which the power density is substantially greater than that of a mixed-combustion membrane fuel cell according to the prior art, and in which the product P (for example from water) is evacuated effectively and without substantial dilution of the compound M1 (eg hydrogen) at the anode or significant dilution of the compound M2 (eg oxygen) at the cathode.

This object is achieved by virtue of the fact that the cathode is traversed by a network of channels, each of which opens on the central membrane and on a free surface of the cathode, the minimum dimension of a cross-section of any one of these channels being greater at 20 μm so that the evacuation of the product P resulting from the reaction of the M ^{(m +)} ions and N ^(n-) ions from the central membrane towards the outside of the fuel cell through these channels is possible.

Thanks to these arrangements, the evacuation of water is more efficient on the one hand because the path of evacuation of water through the channels is shorter, and secondly because the water is evacuated over the entire surface of the channels, a larger area than in the case of the mixed conduction membrane fuel cell according to the prior art where the water is discharged only through the faces of the central membrane which are perpendicular to the interfaces of the central membrane with the first or the second electrolyte. The power density of a battery according to the invention is thus substantially increased.

Advantageously, the first electrolyte and the central membrane consist of the same material which is capable of conducting both the M ^{(m +)} ions and the N ^(n-) ions.

Thus, the manufacture of the fuel cell is greatly simplified, since the assembly consisting of the first electrolyte and the central membrane can be manufactured in a single operation, for example by sintering. The mechanical strength and durability of this assembly are also improved.

• la figure 1 illustre schématiquement une pile à combustible selon l'invention,
• la figure 2 est une vue en coupe selon le plan II-II perpendiculaire à l'axe principal A de la cathode de la pile à combustible de la figure 1,
• la figure 3 est une vue en coupe d'un autre mode de réalisation de la cathode d'une pile à combustible selon l'invention,
• la figure 4 illustre schématiquement une pile à combustible selon l'art antérieur.

The invention will be better understood and its advantages will appear better on reading the detailed description which follows, of an embodiment shown by way of non-limiting example. The description refers to the accompanying drawings in which:

• the figure 1 schematically illustrates a fuel cell according to the invention,
• the figure 2 is a sectional view along the plane II-II perpendicular to the main axis A of the cathode of the fuel cell of the figure 1 ,
• the figure 3 is a sectional view of another embodiment of the cathode of a fuel cell according to the invention,
• the figure 4 schematically illustrates a fuel cell according to the prior art.

• Une anode 10,
• Un premier électrolyte 20,
• Une membrane centrale 30,
• Une cathode 50.

A fuel cell 1 according to the invention is schematically illustrated on the figure 1 . This fuel cell 1, shown in section, comprises four main layers which are in pairs in contact and whose stack along a main axis A is, in this order:

• Anode 10,
• A first electrolyte 20,
• A central membrane 30,
• A cathode 50.

In the description below, and in figure 1 , a heat pump is considered where the first compound M1 is hydrogen H ₂ capable of being oxidized to H ⁺ ions, the second compound is oxygen O ₂ capable of being reduced to O ² ions, and the product P is water H ₂ O.

However, the invention also applies to reactions where the first compound M1 is not hydrogen and the second compound M2 is not oxygen.

Anode 10 is the seat of an oxidation reaction of hydrogen:

{H ₂ → 2H ⁺ + 2 e ^- }.

The H ⁺ protons thus created migrate towards the central membrane 30 through the first electrolyte 20. This first electrolyte 20 is therefore a material capable of conducting the H ⁺ protons.

The e ^- produced electrons flow from the outside of the fuel cell from the anode 10 through a conductor 90 to reach the cathode 50 and supply it with electrons (see below).

The cathode 50 is the seat of an oxygen reduction reaction:

{½ O ₂ + 2 e ^- → O ^2- }

O ^2- ions thus created migrate to the central membrane 30.

Thus a first face 32 of the central membrane 30 is in contact with the first electrolyte 20, and a second face 35 of the central membrane 30, opposite this first face 32, is in contact with the cathode 50.

The first electrolyte 20 is made of a material capable of conducting the protons H ⁺ (or more generally in the case where the first compound is M1, the ions M ^{(m +)} ).

The central membrane 30 is made of a material capable of conducting protons H ⁺ and capable of conducting O ^2- ions (or more generally in the case where the first compound is M 1 and the second compound is M 2, the M ions ^{( m +)} and N ^(n-) ions.

Within this central membrane 30, these protons H ⁺ and these ions O ² react according to the following reaction in order to produce water (or more generally in the case where the first compound is M1 and the second compound is M2, produce a product P):

{2H ⁺ + O ^2- → H ₂ O}

This central membrane 30 is also porous with porosities 38, in order to allow better evacuation towards the cathode 50 of the water thus produced by this central membrane 30.

Advantageously, the first electrolyte 20 and the central membrane 30 consist of the same material which is capable of conducting both the H ⁺ protons (or, in the general case, the M ^{(m +)} ions) and the O ^2- ions (or in the general case N ^(n-) ions).

The manufacture of the fuel cell 1 is thus facilitated.

For example, the material of the central membrane 30 (and the first electrolyte 20) is a ceramic, which has the advantage of controlling its porosity during the manufacture of the fuel cell 1, for example by sintering.

The tests carried out by the inventors have shown that such a mixed conduction ceramic that can be used as material for the fuel cell 1 is, for example, a barium cerate of formula BaCe _0.85 Y _0.15 O ₃ -δ with δ positive.

For example, δ is less than 0.3.

This material is designated BCY15 and has good conduction of both H ⁺ protons and O ^2- ions.

Other materials can be used, for example BaCe _0.5 Zr _0.35 Sc _0.1 Zn _0.05 O _3-δ or BaCe _0.9 Y _0.1 Ru _0.1 O _3-δ with δ positive.

The cathode 50 has within it, that is to say in its volume, channels 52.

Each channel 52 opens at one end on the central membrane 30, and at its other end on a free surface 59 of the cathode 50. These channels 52 may have any geometry, and be in any number.

Advantageously, each channel 52 is rectilinear and extends along the main axis A, that is to say perpendicularly to the central membrane 30.

Each channel 52 is thus perpendicular to the second face 35 of the central membrane 30.

This geometry allows a faster evacuation of water from this second face 35 because the channels then have a minimized length.

For example, the cathode 50 has parallel channels 52 which are each separated from the channel or channels adjacent thereto by a strip 55 of the material of the cathode 50, these strips 55 extending along the main axis A.

For example, as shown on the figure 2 which is a section perpendicular to the main axis A of the cathode 50 of the fuel cell 1 of the figure 1 the cathode 50 is a cylinder whose axis is the main axis A, of circular section. Each channel 52 has a substantially trapezoidal shape, and is separated from the channel or channels adjacent thereto by a strip 55 of the material of the cathode 50.

Alternatively, the cathode 50 has strips 55 which extend along the main axis A and intersect to form a network of parallel channels 52 surrounded by these bands.

For example, as shown on the figure 2 , the strips 55 are divided into two groups, each band 55 being parallel to the bands of its group and perpendicular to the bands of the other group, so as to form an array of parallel channels 52 each of rectangular section (except possibly at the periphery of the cathode 50) and surrounded by four bands 55 (or less than four bands if the channel is at the periphery of the cathode 50). This embodiment of the cathode 50 is represented on the figure 3 .

Alternatively, each of these channels 52 may be of circular section.

In all cases, the minimum size of the channels 52 is at least 20 μm (with 1 μm = 10 ^-6 m).

This minimum size of the channels allows a flow of water.

The minimum size of the channels 52 is greater than that of the pores of the cathode.

The tests carried out by the inventors have shown that the power density of the fuel cell according to the invention is substantially greater than the power density of a mixed conduction membrane fuel cell.

• La formation et l'assemblage du premier électrolyte 20 et de la membrane centrale 30 par une compression à froid et un frittage du matériau constituant ces couches,
• La fixation de l'anode 10 sur le premier électrolyte 20 et de la cathode 50 sur la membrane centrale 30 par un procédé de déposition, par exemple la sérigraphie ou le coulage en bande (en anglais "tape casting"),
• La densification du premier électrolyte 20 lors du frittage,
• L'ajustement de la porosité de la membrane centrale 30.

The inventors have realized a fuel cell 1 according to the invention comprising several layers by combination of the following methods:

• The formation and assembly of the first electrolyte 20 and the central membrane 30 by cold pressing and sintering the material constituting these layers,
• Fixing the anode 10 on the first electrolyte 20 and the cathode 50 on the central membrane 30 by a deposition process, for example screen printing or tape casting (in English "tape casting"),
• Densification of the first electrolyte 20 during sintering,
• The adjustment of the porosity of the central membrane 30.

This manufacturing method facilitates the manufacture of a fuel cell 1 according to the invention.

Densification of the first electrolyte 20 can be achieved by adding a densification agent such as ZnO or CuO during sintering.

The porosity of the central membrane 30 may be made and / or adjusted by the addition of pore-forming additives during sintering, and / or a lower sintering temperature.

The anode 10 and the cathode 50 are for example a ceramic or a cermet (ceramic-metal composite) which are manufactured according to known methods.

• BCY15-Ni/ BCY15 dense/ BCY15 poreux/ BCY15-LSCF
  ou
• BCY15-Ni/ BCY15 dense/ BCY15 poreux/ BCY15-Ag

For example, the composition of a fuel cell 1, where the notation {anode / 1 ^st electrolyte / central membrane / cathode} is used, is one of the following:

• BCY15-Ni / BCY15 dense / BCY15 porous / BCY15-LSCF
  or
• BCY15-Ni / BCY15 dense / BCY15 porous / BCY15-Ag

In the above compositions LSCF designates the ceramic of formula La _1-X Sr _X Co _{1 -Y} Fe _Y O _3-δ with X and Y between 0 and 1, and δ positive.

A fuel cell 1 according to the invention can for example operate at a temperature above 400 ° C, which increases its efficiency.

The invention has been described above in the case of chemical reactions where the first compound M1 is hydrogen and the second compound M2 is oxygen.

The invention is also applicable to other cases, for example fuel cells where the first compound M1 is ethanol, methane or methanol.

In addition, the fuel cell according to the invention has been described above in the case where it operates by electrochemical oxidation-reduction of a first compound M1 and a second compound N2 to produce a compound P and the electricity. The fuel cell according to the invention is also capable of operating in an inverse manner, that is to say as an electrolyzer:
Thus, the compound P is brought into the stack 1 by means of the channels 52 of the cathode 50, and a potential difference is applied to the cell 1. It then triggers within the cell 1 the global chemical reaction.

{p P → n M1 + m N2}

The electrons of the electrical current resulting from this potential difference are taken from the N ^(n-) ions at the cathode 50 (which functions in this case as an anode) and reach the anode 10 (which functions in this case as a cathode where they recombine with the M ^{(m +)} ions.

Such an electrolyzer is therefore able to produce compounds M1 and M2 by electrolysis of compound P. In the case where compound P is H ₂ O, the electrolyser produces hydrogen H ₂ and oxygen O ₂ .

This electrolyser, by the channel structure of the cathode 50, allows a contribution of the compound P more efficiently, and therefore has a better yield.

Claims (10)

1. A fuel cell (1) comprising:

   · an anode (10) suitable for oxidizing a first compound M1 into first ions M^(m+), with m a nonzero integer;

   · a first electrolyte (20) that is in contact with said anode (10);

   · a cathode (50) suitable for reducing a second compound N2 into second ions N^(n-), with n a nonzero integer; and

   · a porous central membrane (30) having one of its faces (32) in contact with said first electrolyte (20), and having its opposite face (35) in contact with said cathode (50);

   said first electrolyte (20) being constituted by a material suitable for conducting the ions M^(m+), said central membrane (30) being constituted by a material suitable for conducting both the ions M^(m+) and the ions N^(n-), said fuel cell being characterized in that said cathode (50) has an array of channels (52) passing therethrough, each channel opening out at said central membrane (30) and at a free surface of the cathode (50), the minimum dimension of a cross-section of any one of the channels (52) being greater than 20 µm, such that it is possible to discharge the product P resulting from the reaction of the ions M^(m+) and the ions N^(n-) from said central membrane (30) to the outside of the fuel cell (1) via said channels (52).
2. A fuel cell according to claim 1, characterized in that said first electrolyte (20) and said central membrane (30) are made of the same material that is suitable for conducting both the ions M^(m+) and the ions N^(n-).
3. A fuel cell according to claim 1 or claim 2, characterized in that said first compound is hydrogen H₂ suitable for oxidizing into H⁺ ions, said second compound is oxygen O₂ suitable for reducing into O^2- ions, and said product P is water H₂O.
4. A fuel cell (1) according to claim 2 or claim 3, characterized in that said material constituting said central membrane (30) is a ceramic.
5. A fuel cell (1) according to claim 4, characterized in that said ceramic is barium cerate having the formula BaCe_0.85Y_0.15O_3-δ, where δ is positive.
6. A fuel cell (1) according to any one of claims 1 to 5, characterized in that each of said channels (52) is rectilinear and extends along said main axis A, i.e. perpendicularly to said central membrane (30).
7. A fuel cell (1) according to claim 6, characterized in that said cathode (50) presents parallel channels (52), each separated from the adjacent channel(s) by a respective strip (55) of the material of the cathode (50), these strips (55) extending along the main axis A.
8. A fuel cell (1) according to claim 6, characterized in that said cathode (50) presents strips (55) that extend along the main axis A and cross one another to form an array of parallel channels (52) surrounded by the strips.
9. A fuel cell (1) according to claim 8, characterized in that each of said channels (52) is of rectangular section.
10. A fuel cell (1) according to claim 8, characterized in that each of said channels (52) is of circular section.

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